The three major groups of freshwater zooplankton

The three major groups of freshwater zooplankton

This section deals with general aspects of zooplankton taxonomy. The focus is on the three most common types of lake zooplankton -- rotifers, cladocerans, and copepods. The descriptive information provided below is adapted from Thorp, J.H. and Covich, A.P. (1991). Ecology and Classification of North American Freshwater Invertebrates. Academic Press, NY. If you are interested in detailed information about zooplankton taxonomy, biology, and ecology, I recommend this book as your primary reference.

Rotifers (Phylum Rotifera)

These are small zooplankton, typically 100 to 500 um (microns) in length, with two distinctive features. First, they have a ciliated region at the apical or head end called a corona. This is used for locomotion and for gathering of food particles from the water. Second, rotifers have a muscular pharynx (mastax) made of two hard jaws called trophi. These tiny structures are used to identify rotifers to the species level. When a rotifer is viewed from the anterior end, the corona makes it look like a rotating wheel. That is the basis for the name of this Phylum. There are various taxonomic keys that can be used to identify freshwater rotifers, of which there are more than a thousand species. A good key for species commonly found in lake plankton is Stemberger, R.S. 1979. A Guide to Rotifers of the Laurentian Great Lakes. US Environmental Protection Agency (available from the National Technical Publication Service, PB80-101280).

Photos of four common rotifers: Brachionus, Conochilus (colony), Asplanchna, and Lecane.

Cladocerans (Phylum Arthropoda, Subphylum Crustacea, Order Cladocera)

Cladocerans include species ranging in size from less than 500 um (microns) to over 1 mm. They commonly are referred to as "water fleas," and perhaps the best known genus is Daphnia. Cladocerans are microscopic crustaceans that have a single compound eye and a bivalve carapace that functions as a brood chamber. In most species the carapace wraps around the entire body except the head. Four to six pairs of thoracic legs beat in rhythm to generate a current that carries food particles to the animal's mouth. The first pair of legs (antennae) are enlarged and are used for swimming. Cladocerans are generally identified on the basis of body shape and size, and specifically identified by examining an abdominal structure called the postabdominal claw. Larval cladocerans develop inside the maternal brood chamber and when they are released, they closely resemble the adult. Thorp and Covich (cited above) provide a good general key for the Cladocera. There also exist keys for particular genera, such as Daphnia (Brooks, J.L.1957. The Systematics of North American Daphnia. Yale University Press).

Photos of three common cladocerans, from left:Eubosmina, Daphnia, and Diaphanosoma.

Copepods (Phylum Arthropoda, Subphylum Crustacea, Order Copepoda)

Copepods range in size from less than 500 um (microns) to over 2 mm in length. They have a cylinder-shaped segmented body with many small appendages on the head and thorax and two caudal setae at the end of the abdomen. They have a prominent exoskeleton, a large pair of first legs (antennae) that are used for swimming, and a single eye. The two most common types of copepods in lakes and ponds are Calanoida (calanoids) and Cyclopoida (cyclopoids). Most copepods are transparent or gray/brown in color, although it is common to observe bright red and orange colors due to accumulated lipid droplets inside the body. Copepods feed on a wide range of food, ranging from small zooplankton to algae and bacteria. They are truly omnivores when one considers their position in the food web. Female copepods carry their eggs externally. Eggs hatch into a larval stage called a nauplius (pleural nauplii), which grows through six stages, followed by six copepodite stages. There is a pronounced metamorphosis between the nauplius and copepodid stages, with the latter having an appearance similar to the adults. There are numerous taxonomic keys for freshwater copepods. Thorp and Covich (cited above) provide a good general key. For North American taxa the following key is useful for identification to the species level: Smith, K. and C.H. Fernando. 1978. A Guide to the Freshwater Calanoid and Cyclopoid Copepod Crustacea of Ontario. Department of Biology, University of Waterloo, Ontario, Canada.